An increase in sensitivity to the pressor effects of tyramine, an indirectly acting sympathomimetic agent, is a well known side effect of MAO inhibitor treatment. Using an intravenous tyramine steady state infusion technique we have studied the effects of the selective inhibition of the A and B forms of MAO on tyramine's pressor effects in patients treated with clorgyline, pargyline and deprenyl. Our data have demonstrated that treatment with clorgyline is associated with the greatest increases in tyramine sensitivity, while treatment with pargyline is associated with a lesser increase in tyramine sensitivity. Deprenyl treatment was associated with a negligible alteration in response to tyramine. This data suggests that the tyramine sensitivity which develops during MAO inhibitor administration is principally due to a reduction in MAO type A activity. A high correlation observed between changes in tyramine sensitivity and reduced plasma 3-methoxy,4-hydroxy phenylglycol concentrations supports this interpretation.